Find the mass number and atomic number of the daughter after alpha decay of uranium-238.

Solution & explanation

1. Uranium-238 has A = 238 and Z = 92. Alpha has A = 4 and Z = 2.

2. Conserve nucleon number.

   $$A_d=238-4=234$$

3. Conserve proton charge number.

   $$Z_d=92-2=90$$

4. Atomic number 90 is thorium.

5. Write ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He + released energy.

6. Check: 234 + 4 = 238 and 90 + 2 = 92.

Daughter: thorium-234, A = 234, Z = 90